The present invention relates to an LNG power plant which uses liquefied natural gas (LNG) as fuel, and an operation method of the LNG power plant.
A general gas turbine power generation facility has a combustor. The combustor mixes fuel with compressed air, and burns the fuel to generate high-temperature and high-pressure combustion gas. This combustion gas is introduced into a gas turbine to rotate a rotor. Rotation energy of the rotor is transmitted to a power generator to generate electric power.
Liquefied natural gas (LNG) widely known as clean energy source is often used as the fuel introduced into the combustor. An LNG vaporization facility vaporizes LNG as fuel gas.
In a gas turbine power plant, an LNG vaporization facility and a gas turbine power generation facility are often constructed separately. Therefore, there is no linked operation between both facilities, and they are operated individually. For example, Japanese Patent No. 3214709, Japanese Patent Application Publication Hei 2-240499, Japanese Patent Application Publication 2001-124295, Japanese Patent Application Publication 2002-115564, Japanese Patent Application Publication 2002-188460 and Japanese Patent Application Publication 2003-49718 describe control devices and methods of the LNG vaporization facility, but those documents do not disclose control devices or methods of the power generation facility.
A compact power plant constructed recently has an LNG vaporization facility adjacent to a power generation facility.
The compact power plant has a thermally insulated LNG storage tank installed in the LNG vaporization facility. A boost pump pressurizes LNG stored in the LNG storage tank, and sends the LNG to a vaporizer. The vaporizer vaporizes the LNG.
Even if the LNG storage tank is insulated, heat entry into the LNG storage tank is inevitable. Hence, a slight quantity of gas constantly vaporizes from the stored LNG. This gas is called a boil off gas (BOG).
The BOG generated in the LNG storage tank is discharged through a BOG outlet valve. The BOG discharged from other LNG storage tanks is collected together and is pressurized by a BOG compressor. Thereafter, the BOG is mixed with the LNG outputted from the vaporizer by a gas feed pipe, and fed to the power generation facility.
Most of LNG used in Japan is imported and the characteristics of LNG vary depending on the origin of the LNG. So, the compositions of the LNG stored in different LNG storage tanks may differ from each other. This difference causes a difference in heating values.
Therefore, the power generation facility is equipped with a calorimeter or gas chromatography on the gas feed pipe to detect or calculate a heating value of the gas and control the gas flow rate of the LNG vaporization facility to keep the heating value of the gas in a small range.
In addition, the power generation facility is equipped with a gas heating/cooling device on the gas feed pipe connected to a fuel adjust valve. Temperature of the gas at the outlet of the gas heating/cooling device is measured and its signal is sent to the gas heating/cooling device. The gas heating/cooling device controls the temperature of the fuel gas to keep the temperature of the fuel gas introduced into the gas turbine appropriate.
In a power generation plant in which the LNG vaporization facility and the power generation facility are constructed adjacent to each other to be compact, there may not be enough space to install a processing device for generation of the BOG in the LNG vaporization facility.
A quantity of BOG varies over a very wide range. For example, when receiving LNG, a large quantity of BOG is generated. During winter when the air temperature is low, BOG quantity is smaller.
Therefore, based on a pressure of a LNG storage tank, devices, such as a BOG outlet valve and a BOG compressor, control BOG flow rate to prevent excessive pressure rise in the LNG storage tank. As a result, the change in the BOG generation rate influences the gas turbine of the power generation facility directly.
A gas turbine has a limit about a fuel. This limitation is about a value called a Wobbe Index (a function of heating value, specific gravity of fuel and combustion temperature) expressed by the expression (1).
                              Wobbe          ⁢                                          ⁢          Index                =                  LHV                                                    Mwgas                28.96                                      ⨯            Tgas                                              (        1        )            where                LHV: Lower Heating Value        Mwgas: molecular weight of the fuel gas        Tgas: absolute temperature of the fuel gas (degree Rankine)        
Heating value of BOG is different from the heating value of LNG. Therefore, as BOG flow rate changes, characteristics of the fuel quality changes rapidly. Consequently, calorie of the fuel flowing into the gas turbine of the power generation facility varies.
If calorie of fuel varies frequently, the calorie may not remain within a limitation range of the Wobbe Index (e.g., ±5%). Therefore, it is necessary to add facilities such as a flare facility which externally burns the BOG and/or a tank which adjusts the flow rate of a fuel gas in the middle of the gas feed pipe. In addition, if the mixture rate of the BOG in the fuel gas rises close to 100%, it is difficult to operate the gas turbine.
If the index of the fuel index deviates from the limitation range even after taking these countermeasures, operation of the power plant has to be stopped.